CS5165 datasheet(11/21 Pages) ONSEMI

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CS5165

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Start Up

Until the voltage on the VCC supply pin exceeds the 3.95 V

monitor threshold, the Soft Start and GATE pins are held

low. The FAULT latch is reset (no Fault condition). The

output of the error amplifier (COMP) is pulled up to 1.0 V by

the comparator clamp. When the VCC pin exceeds the

monitor threshold, the GATE(H) output is activated, and the

Soft Start capacitor begins charging. The GATE(H) output

will remain on, enabling the NFET switch, until terminated

by either the PWM comparator, or the maximum on time

timer.

If the maximum on time is exceeded before the regulator

output voltage achieves the 1.0 V level, the pulse is

terminated. The GATE(H) pin drives low, and the GATE(L)

pin drives high for the duration of the extended off time. This

time is set by the time out timer and is approximately equal

to the maximum on time, resulting in a 50% duty cycle. The

GATE(L) pin will then drive low, the GATE(H) pin will

drive high, and the cycle repeats.

When regulator output voltage achieves the 1.0 V level

present at the COMP pin, regulation has been achieved and

normal off time will ensue. The PWM comparator

terminates the switch on time, with off time set by the COFF

capacitor. The V2 control loop will adjust switch duty cycle

as required to ensure the regulator output voltage tracks the

output of the error amplifier.

The Soft Start and COMP capacitors will charge to their

final levels, providing a controlled turn on of the regulator

output. Regulator turn on time is determined by the COMP

capacitor charging to its final value. Its voltage is limited by

the Soft Start COMP clamp and the voltage on the Soft Start

pin.

Power Supply Sequencing

The CS5165 offers inherent protection from undefined

start up conditions, regardless of the 12 V and 5.0 V supply

power up sequencing. The turn on slew rates of the 12 V and

5.0 V power supplies can be varied over wide ranges without

affecting the output voltage or causing detrimental effects to

the buck regulator.

Figure 10. Demonstration Board Startup in Response

to Increasing 12 V and 5.0 V Input Voltages. Extended

Off Time is Followed by Normal Off Time Operation

when Output Voltage Achieves Regulation to the Error

Amplifier Output.

M 250 μs

Trace 3− 12 V Input (VCC) (5.0 V/div.)

Trace 1− Regulator Output Voltage (1.0 V/div.)

Trace 4− 5.0 V Input (1.0 V/div.)

Trace 2− Inductor Switching Node (2.0 V/div.)

Figure 11. Demonstration Board Startup Waveforms

Trace 2− COMP PIn (error amplifier output) (1.0 V/div.)

Trace 1− Soft Start Pin (2.0 V/div.)

Trace 4− Regulator Output Voltage (1.0 V/div.)

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